The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
As it is known per se, an aircraft propulsion unit conventionally comprises a turbojet engine housed inside a nacelle.
The nacelle generally has a tubular structure comprising an air inlet upstream of the turbojet engine, a median section intended to surround a fan of the turbojet engine and the casing thereof, and a downstream section intended to surround the combustion chamber of the turbojet engine and accommodating if necessary thrust reversal means. It may be ended by an ejection nozzle whose outlet is located downstream of the turbojet engine.
In order to reduce the noise pollution generated by the turbojet engine, it is common to equip several parts of the nacelle and/or the turbojet engine with acoustic attenuation structures.
These parts of the nacelle and/or of the turbojet engine might be, for example, an inner surface of an air circulation flow path for a bypass turbojet engine, a nozzle or ejection cone surface, etc.
An acoustic attenuation structure has the shape of an acoustic panel of the alveolar-core sandwich type comprising, at least one resonator, for example of the honeycomb type, covered with an outer perforated skin (called acoustic skin) and with a solid inner skin (called support or structuring skin).
One and/or the other of these skins might be made of a composite material comprising a set of fibers (superposition of fibrous plies), embedded in a polymerized resin.
The inner and outer skins are surfaces of the panel which are exposed to the air flow circulating through the propulsion unit and may be damaged, and in particular undergo tearing, perforations, detachments for example at the skin/honeycomb acoustic structure interfaces.
It is therefore appropriate to repair the panel accordingly damaged so as to restore to the panel the properties thereof, in terms of acoustic performance and structural strength.
In case of a damage of a skin of the panel, said skin might be repaired by using a doubler, that is to say a skin portion, metallic or composite for example, applied in superposition on the skin to be repaired at the damaged zone.
The use of such doublers has several drawbacks.
In the first place, the use of a doubler, and more particularly a metallic doubler, adds a certain mass to the panel, which is not desirable.
Also, some panels may have particularly complex geometric structures due, for example, to their location within the propulsion unit (high curvature, unconventional shape, etc.). Thus, it is desired to prepare the doubler via a prior shaping corresponding to the geometry of the panel to be repaired in order to facilitate the laying thereof and to restore the properties of said panel at best.
Such a prior operation is long, complex and expensive.
It is also known to place the whole damaged part in order to be able to repair it in the workshop. The repair is carried out for example by vacuum molding of the doubler on the damaged part by means of a bladder including the entire damaged part, in order to manufacture the doubler made of composite material. This operation is long, expensive and delicate.
Although such a repair is effective, the removal and the laying of the whole part to be repaired are time-consuming and delicate.
In the case where the use of a doubler is difficult or these drawbacks are too significant, there will be no other solution but entirely replacing the damaged panel, which is of course not desirable.